Impact resistant nickel-chromium alloys



United States Patent ABSTRACT OF THE DISCLOSURE The impact resistance of certain nickel-chromium alloys of specified composition is enhanced through control of carbon content. In addition to nickel, chromium and carbon, the alloys contain tungsten, aluminum, Zirconium and boron, molybdenum and columbium being optional.

The present invention relates to nickle-base alloys and, more particularly, to cast nickel-chromium base alloys having improved impact resistance characteristics together with good stress-rupture properties at elevated temperatures.

As is well known by those skilled in the art, extensive and continuous efforts have been made in providing new and improved materials capable of meeting the increasingly stringent requirements imposed by high temperature application. For example, applications involving stator and rotor blades for gas turbine engines necessitate the use of alloys capable of withstanding high stress at elevated temperatures up to 1000 C. and above. In this connection, notable advances have been achieved and in our US. Patent No. 3,166,413 we have described various alloys having a highly satisfactory combination of stressrupture properties and high temperature ductility, the alloys containing the following constituents within the given ranges: about 5% to chromium, about 7% to 16% tungsten, up to about 5% molybdenum, up to about 4% columbium (niobium) with the sum of the percentages of tungsten, molybdenum and columbium plus two-thirds the percentage of chromium being about 17.5% to 20.5%, about 2% to 8% aluminum, about 0.03% to 0.3% carbon, up to 1% zirconium, up to 0.05% boron, up to 3% total of iron, manganese and silicon (principal impurities), up to 1% cobalt, the balance of the alloys-being essentially nickel and impurities.

However, as is further known by those skilled in the art, it would be most desirable for various high temperature applications to have at hand alloys which manifest a high degree of resistance to impact, i.e., the ability to absorb a high level of energy, as well as having good stress-rupture characteristics. Now, the alloys described in our aforementioned patent afford good resistance to impact but we have found this characateristic can be markedly improved by controlling the carbon content of the alloys.

It is an object of the present invention to provide a new and improved alloy useful for high temperature application.

Another object of the invention is to provide a novel, nickel-base alloy characterized by a high degree of resistance to impact at elevated temperature.

The invention also contemplates providing novel cast nickel-base alloys which afford a good level of impact resistance and other mechanical properties at high temperatures, e.g., 850 C. and above.

Other objects and advantages will become apparent from the following description.

In accordance with the present invention, the carbon content of the above-described alloys is controlled such that it is less than 0.03%, e.g., 0.001% to 0.02'/5%. The carbon content is preferably as low as possible, e.g., less than 0.02% or even less than 0.01%, though a trace of carbon will inevitably be present. To achieve the best stress-rupture lives, the chromium content should not exceed 9% and, advantageously, is present in an amount of from 5% to 7%. As set forth in Our US. patent above identified, the stress-rupture life also depends on the total content of tungsten, molybdenum and columbium and at a given chromium content there is an optimum value of this total at which the longest lives are obtained. In alloys containing 6% chromium, the longest lives are achieved when the total tungsten plus molybdenum plus columbium is about 15%. With increasing chromium contents, the optimum total tungsten plus molybdenum plus columbium decreases and in alloys containing 9% chromium, it is 13%.

Preferably both molybdenum and columbium are present in the alloys in amounts not exceeding 4% and 2.5%, respectively, and the tungsten content is from 9% to 14%.

The aluminum content of the alloys is important and in alloys of any given base composition variation of aluminum content has a marked effect on stress-rupture life. The longest life is obtained with aluminum contents in a range of 5% to 7% and alloys with aluminum contents within this range are therefore particularly suitable for gas turbine rotor blades which require the best possible stress-rupture properties. At lower aluminum contents, the alloys have higher melting points and alloys with from 5% down to 3.5% or even 2% aluminum are suitable for parts such as gas turbine stator blades which require a high melting point but are less highly stressed than rotor blades.

In carrying the invention into practice and particularly to obtain alloys with low carbon contents, we prefer to employ vacuum melting. Under these conditions, carbon reacts with oxides introduced by the charge materials and is substantially eliminated as carbon monoxide. Whether or not they are vacuum melted, the alloys are advantageously subjected to a vacuum refining treatment comprising holding them in the molten state under high vacuum before casting the melt. We prefer to hold the melt at a temperature of 1400 C. to 1600 C. at not more than microns pressure for a period of at least 15 minutes and advantageously for 60 minutes or more. The duration of the treatment depends to some extent on the purity of the ingredients of the melt, a longer time being required when less pure ingredients are employed.

When making small castings, for example, turbine blades or stress rupture testpieces, the alloys are preferably cast under vacuum but when making large castings from a melt that has been produced or refined under vacuum, it makes little difference to the properties obtained whether casting is carried out in vacuum, inert gas or air.

For the purpose of giving those skilled in the art a better understanding and/or appreciation of the invention, the following comparative tests are given to illustrate the improvement in impact resistance at elevated temperature (850 C.) that results in controlling the amount of carbon present in the alloys. In this connection, Alloy No. 1' is in accordance with our US. Patent No. 3,166,413, whereas Alloy No. 2 is in accordance-with the present invention.

The tests were performed on test pieces machined from as-cast specimens of the alloys which were prepared by melting under vacuum charges consisting of nickel, chromium, molybdenum, tungsten and, in the case of Alloy No. 1, an addition of carbon, and holding the melt under a pressure of less than microns until the car- The effect of zirconium and boron is set forth in Table hon-oxygen reaction had virtually ceased. Additions of III.

TABLE III Composition (percent by weight) Impact Stress-rupture Alloy Value at t.s.i./950 C. No. C Cr Mo W Cb Al Zr 13 850 C., Lite,(hrs.)

(rt-1b.)

s 0.002 6 2 11 1.5 0 0.25 0.02 so 01 columbium, aluminum, zirconium and boron were then In most instances, the above results represent the avermade to the molten bath and the melt was refined by 15 age of two specimens. Alloys Nos. 1 and 7 through 9 holding for minutes at 1500 C. under a pressure of illustrate the effect of zirconium, Alloy No. 1, of course, less than one micron. The balance of the composition of being reproduced from above and being outside the inveneach alloy was nickel. tion in view of the amount of carbon. As a practical mat- TABLE 1 Impact Stress-Rupture Composition (percent by weight) Value at Properties 15 Alloy 850 C. t.s.i./950 o.

No. (ft.-ib.)

C Cr Mo W Cb Al Zr 13 Life, EL,

(hr.) (percent) 1 0.06 s 2 11 1.5 s 0.12 0.02 33, 30 75 N.D. 2 0. 001 6 2 11 1.5 0 0.12 0.02 32, 91 5s, 03 4.6,N.D.

1 Analyzed carbon contents on impact and stress-rupture bars. N.D .=Not Determined.

The foregoing data illustrates the striking improvement 30 ter, the combined impact. resistance and stress-rupture achieved regarding Alloy No. 2 in comparison with Alloy life of each of Alloys Nos. 7, 8 and 9 was significantly No. 1. The decrease in stress-rupture life is far outweighed superior to Alloy No. 1, the stress-rupture life of Alloys by the exceptional improvement in impact resistance, the Nos. 7 and 8 in particular being comparable to Alloy No. magnitude of improvement being of the order of 150%. 1. Alloy No. 10 indicates that it is advantageous to main- In any event, as will be illustrated herein, we have further tain the zirconium level below 0.7%. A comparison of found that loss of stress-rupture life attributable to low Alloys Nos. 9 and 11 reflects the attributes of using boron carbon content can be greatly minimized if not completely in amounts contemplated herein. eliminated through control of the elements boron and The alloys according to the invention are particularly zirconium. suitable for making cast rotor blades for gas turbine en- While carbon exerts a remarkable influence regarding gines. Articles and parts cast from the alloys may be used the resistance to impact of the alloys of the invention, it in the as-cast condition for high temperature service. If does not necessarily at all follow that this influence manidesired, the alloys may be homogenized by heating in the tests itself regarding other nickel-chromium base alloys, temperature range 850 C to 125 C. before being put even those of compositions which might be considered into service. somewhat similar to alloys contemplated herein. This is For use at temperatures above 1000 C. under condiillustrated by the data given in Table II regarding pairs tions such as are encountered in gas turbine engines, inof Alloys Nos. 3 and 4 and 5 and 6. volving both oxidation and sulfur attack, articles and parts TABLE 11 Composition 1 (percent by weight) Impact Stress-rupture Alloy N0. Value at 15 t.s.i./950 C.

Cr C0 Mo v Ti Al Zr 850 0., Lite, (hrs) (it-1b.)

1 All alloys nominally contained 0.015% boron, balance nickel. Alloys Nos. 3 and 4 are of the same composition except for the carbon content. The same applies to Alloys Nos. 5 and 6 but it is to be observed that the alloys of low carbon (Nos. 4 and 6) did not afford any appreciable improvement in impact resistance. The low carbon 0! Alloy No. 4 drastieally adversely affected the stress-rupture life.

As noted above, in respect of Alloy No. 2, a decrease of stress-rupture life is experiencedwith lower carbon contents. While the loss would not preclude use of the alloys for a host of applications, particularly in view of the magnitude in improvement regarding impact resistance, such loss can be greatly minimized provided the alloys contain at least 0.01% and up to 1% zirconium and at least 0.003% and up to 0.075% or 0.08% boron. Advantageously, the alloys contain about 0.05 to 0.7% zirconium and about 0.01% to 0.05% boron, and it is further preferred that the amounts of zirconium and boron be correlated such that the value of the expression:

tective coating, for example, of aluminum.

35 conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims. However, as set forth in our U.S. patent referred to herein, and for reasons given therein, the alloys should not contain vanadium, should be substantially free Percent zirconium+10 percent boron is about of cobalt, i.e., any cobalt should not exceed 1%, and

0.2% to 0.7% titanium is an undesirable impurity and should not be made from the alloys are preferably provided with a pro- Although the present invention has been described in e present in an amount exceeding 0.5% and, advantageously, it is less than 0.25

We claim:

1. An alloy characterized by a combination of good impact resistance and stress-rupture life at elevated temperatures, said alloy consisting essentially of about 5% to 10% chromium, about 7% to 16% tungsten, up to about 5% molybdenum, up to about 4% columbium with the sum of the percentages of tungsten, molybdenum and columbium plus two-thirds the percentage of chromium being about 17.5% to 20.5%, about 2% to 8% aluminum, carbon in an amount up to less than 003%, about 0.01% to 1% zirconium, about 0.003% to 0.08% boron and the balance being essentially nickel.

2. A cast alloy characterized by a combination of good impact resistance and stress-rupture life at elevated temperatures, said alloy consisting essentially of about 6% chromium, about 11% tungsten, about 2% molybdenum, about 1.5% columbium, about 6% aluminum, about 0.004% carbon, about 0.25% zirconium, about 0.02%

boron, and the balance being essentially nickel.

3. A cast nickel-chromium-base alloy characterized by a combination of good impact resistance and stress-rupture life at elevated temperatures, said alloy being suitable for use in turbine structures and characterized by good impact resistance and stress-rupture properties at relatively high stress and elevated temperatures, said alloy consisting essentially of about 5% to 10% chromium, about 7% to 16% tungsten, up to about 5% molybdenum, up to about 4% columbium, the sum of the percentages of tungsten, molybdenum and columbium and two-thirds the percentage of chromium being about 17.5% to 20.5%, about 2% to 8% aluminum, about 0.001% to 0.0275% carbon, about 0.05% to about 0.7% zirconium, about 0.01% to about 0.05% boron up to not more than 0.5% titanium, up to not more than 1% cobalt, and the balance essentially nickel.

4. A cast alloy in accordance with claim 3 containing about 5% to 7% aluminum, about 0.001% to 0.02% carbon, about 5% to 9% chromium, about 9% to 14% tungsten, up to 4% molybdenum and up to 4% columbium.

References Cited UNITED STATES PATENTS 3,160,500 12/1964 Eiselstein et a1 75171 3,166,412 1/1965 Bieber 75l71 3,166,413 1/1965 Shaw et al. 75-l71 DAVID L. RECK, Primary Examiner. RICHARD O. DEAN, Examiner. 

